As mobile devices have been increasingly developed, and the demand for such mobile devices has increased, the demand for secondary batteries has also sharply increased as an energy source for the mobile devices. Among such secondary batteries is a lithium secondary battery having high energy density and high discharge voltage, on which much research has been carried out and which is now commercialized and widely used.
A secondary battery has attracted considerable attention as an energy source for power-driven devices, such as electric bicycles (E-bike), electric vehicles (EV), or hybrid electric vehicles (HEV), as well as an energy source for mobile wireless electronic devices, such as mobile phones, digital cameras, personal digital assistants (PDA), portable multimedia players (PMP), and laptop computers.
A small-sized battery pack having a battery cell packed therein is used for small-sized devices, such as mobile phones and digital cameras. On the other hand, a middle or large-sized battery pack including two or more battery cells (hereinafter, occasionally referred to as a “multi-cell”) connected in parallel and/or in series to each other is used for middle or large-sized devices, such as laptop computers and electric vehicles.
As previously described, a lithium secondary battery exhibits excellent electrical properties; however, the lithium secondary battery has low safety. For example, when abnormal operations, such as overcharge, overdischarge, exposure to high temperature, and electrical short circuits, of the lithium secondary battery occur, decomposition of active materials and an electrolyte, which are components of the battery, is caused, with the result that heat and gas are generated, and the high-temperature and high-pressure condition caused by the generation of the heat and the gas accelerates the above-mentioned decomposition. Eventually, a fire or explosion may occur.
For this reason, the lithium secondary battery is provided with a safety system, such as a protection circuit for interrupting electric current during overcharge, overdischarge, or overcurrent of the battery, a positive temperature coefficient (PTC) element whose resistance greatly increases so as to interrupt electric current when the temperature of the battery increases, and a safety vent for interrupting electric current or discharging gas when pressure increases due to the generation of the gas. In the case of a small-sized cylindrical secondary battery, for example, the PTC element and the safety vent are usually disposed at the top of an electrode assembly (a generating element) having a cathode/separator/anode structure, which is mounted in a cylindrical case. In the case of a prismatic or pouch-shaped small-sized secondary battery, on the other hand, the protection circuit module and the PTC element are usually mounted at the upper end of a prismatic case or a pouch-shaped case, in which the generating element is mounted in a sealed state.
The safety-related problem of the lithium secondary battery is even more serious for a middle or large-sized battery pack having a multi-cell structure. Since a plurality of battery cells are used in the multi-cell structure battery pack, the abnormal operation of some of the battery cells may cause the abnormal operation of the other battery cells, with the result that a fire or explosion may occur, which may lead to a large-scale accident. For this reason, the middle or large-sized battery pack is provided with a safety system, such as a fuse, a bimetal, and a battery management system (BMS), for protecting the battery cells from the overcharge, the overdischarge, and the overcurrent.
However, as the lithium secondary battery is continuously used, i.e., as the lithium secondary battery is repeatedly charged and discharged, the generating element and the electrically connecting members are gradually degraded. For example, the degradation of the generating element leads to the decomposition of the electrode material and the electrolyte, by which gas is generated. As a result, the battery cell (the cylindrical, prismatic, or pouch-shaped case) gradually swells. In the normal state of the lithium secondary battery, the safety system, i.e., the BMS, detects the overdischarge, the overcharge, and the overcurrent, and controls/protects the battery pack. In the abnormal state of the lithium secondary battery, however, when the BMS does not operate, a possibility of danger increases, and it is difficult to control the battery pack for securing the safety of the battery pack. The middle or large-sized battery pack is generally constructed in a structure in which a plurality of battery cells is fixedly mounted in a prefabricated case. As a result, the respective swelling battery cells are further pressurized in the restrictive case, and therefore, a possibility of a fire or explosion greatly increases under the abnormal operation condition of the battery pack.
In connection with this case, FIG. 1 is a circuit diagram typically illustrating a conventional middle or large-sized battery pack. Referring to FIG. 1, a conventional middle or large-sized battery pack 900 includes a battery module assembly 500 including a plurality of battery modules electrically connected to each other, each of the battery modules including a plurality of battery cells or unit modules connected in series to each other while being mounted in a module case, a BMS 600 for detecting information on the operation status of the battery module assembly 500 and controlling the battery module assembly 500 based on the detected information, and a power switch unit (relay) 700 for performing connection or disconnection between the battery module assembly 500 and an external input and output circuit (inverter) 800 according to an operation command from the BMS 600.
The BMS 600 keeps the power switch unit 700 on during normal operation of the battery module assembly 500 and turns the power switch unit 700 off, when it is detected that the operation of the battery module assembly 500 is abnormal, to stop charge and discharge operations of the battery module assembly 500. During malfunction or non-operation of the BMS 600, on the other hand, the BMS 600 performs no control, and therefore, the power switch unit 700 is kept on. As a result, charge and discharge operations of the battery module assembly 500 are continuously performed even during abnormal operation of the battery module assembly 500.
Therefore, there is a high necessity for technology that is capable of fundamentally securing safety of a middle or large-sized battery pack while solving the above problems.